1. Field of the Invention
The present invention relates to an imaging apparatus configured to capture an object image.
2. Description of the Related Art
An imaging apparatus such as a digital still camera and a video camera generally uses, as its image sensor, a charge-coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor.
As a characteristic of the above-described image sensor, a fixed pattern noise due to an uneven dark current in a photodiode per each pixel and a defect pixel due to a dark current can occur. These phenomena degrades a quality of a captured image.
The level of a dark current is generally said to rise twice as high when a temperature of a surface of an image sensor rises by 8° C. That is, the fixed pattern noise due to an uneven dark current in a photodiode per each pixel and the defect pixel due to a dark current have temperature characteristics. Accordingly, fixed pattern noises or defect pixels occur more frequently when the temperature of the surface of an image sensor rises.
Japanese Patent Application Laid-Open No. 01-147973 discusses a method for suppressing a fixed pattern noise caused by a dark current in a photodiode that has the temperature characteristics as described above. More specifically, Japanese Patent Application Laid-Open No. 01-147973 discusses a method for correcting a fixed pattern noise by previously storing a fixed pattern noise per each predetermined temperature range, detecting a temperature of an image sensor, reading a fixed pattern noise corresponding to the detected temperature, and subtracting the fixed pattern noise from a signal of an actual captured image.
In addition, a conventional method stores an address of a defect pixel caused by a dark current and interpolates defect pixel information based on information about pixels around the defect pixel. Furthermore, defect pixels can also be corrected by previously storing an address of a defect pixel for each predetermined temperature range, similar to Japanese Patent Application Laid-Open No. 01-147973.
Moreover, another conventional method performs an imaging operation immediately before or after the actual imaging operation, in a state that an entire image sensor is shielded from light. Thus, a captured image including only dark current elements is obtained and subtracted from a signal of an actual captured image. As a result, degradation of an image quality due to a fixed pattern noise caused by a dark current which includes a temperature variation factor, or defect pixels arising from a dark current, can be reduced or suppressed (correction by an actual dark subtraction).
Further, Japanese Patent Application Laid-Open No. 2001-174329 discusses a method for suppressing or reducing a fixed pattern noise by detecting a temperature around an image sensor, based on an average value in an optical black area of the image sensor. The method discussed in Japanese Patent Application Laid-Open No. 2001-174329 is highly useful in the case where the entire image sensor shows a uniform temperature variation.
However, it is possible that only the temperature of a part of an image sensor rises because of a partial temperature rise in inner circuits within the image sensor or peripheral circuits around the image sensor. In such a case, the dark current level in the portion of the image sensor where temperature is high is different from the portion of the image sensor where temperature is low. Accordingly, a fixed pattern noise cannot be appropriately and effectively suppressed or reduced. Furthermore, in this case, an image quality of a captured image can degrade due to uneven brightness or uneven color arising from a difference in dark current levels on the surface of the image sensor.
In addition, in the above case where the temperature is uneven within (on the surface of) the image sensor, if the entire surface of the image sensor is corrected according to a defect pixel address for each predetermined temperature range as in the method discussed in Japanese Patent Application Laid-Open No. 01-147973, a portion of the image can be excessively corrected or insufficiently corrected.
In most cases, defect pixels are corrected by interpolating defect pixel information with information about surrounding pixels. In the case where the image is excessively corrected, when a minute pattern is captured, the reproducibility of the pattern generally degrades. In the case where the image is insufficiently corrected, noises can more distinctly appear in the image, which causes image degradation.
In recent years, an image sensor has a large number of pixels, and accordingly, a chip size of an image sensor has also become larger. Some of imaging apparatuses such as a digital still camera, which are required from the market to achieve a high image quality, have a chip size equal to an image taking area of a 35 mm film in a silver halide camera. In such a large-sized image sensor, uneven brightness, uneven color, and defect pixels become increasingly apparent.
Furthermore, in making correction by the actual dark subtraction, two shootings are performed when one scene is captured. Thus, a time taken for capturing one scene becomes twice as long. Accordingly, a frame rate in a video camera and continuous shooting performance of a digital still camera can considerably degrade.